1. Field of the Invention
The present invention relates to an imaging device for forming an image of a recording medium, and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus of the related art, for example, the user sets the type (e.g., size or thickness) of a recording medium by the use of an external apparatus, such as a computer, or a control panel provided on a main body of the image forming apparatus. According to the setting, for example, transfer conditions (a transfer voltage and a conveying speed of the recording medium during transfer) and fixing conditions (a fixing temperature and a conveying speed of the recording medium during fixing) are controlled.
To reduce this user's burden of setting the type of the recording medium by the use of the computer or the control panel, there has been proposed an image forming apparatus that automatically identifies the type of a recording medium with a sensor or the like incorporated therein. This image forming apparatus equipped with the sensor or the like exerts control so that the type of the recording medium is automatically identified and conditions of transfer, fixing, etc. are set according to the identification result.
More specifically, Japanese Patent Laid-Open Nos. 2002-182518 and 2004-038879 propose that the type of a recording medium is identified by imaging a surface of the recording medium with a CMOS sensor and detecting the surface smoothness from an obtained image. When imaging is thus performed with the CMOS sensor, since shadows due to surface irregularities are directly imaged, the recording medium can be identified accurately. An accuracy is high particularly when identifying the type of a recording medium in which the presence, size, and depth of surface irregularities can be visually and clearly recognized, for example, when distinguishing between coated paper and non-coated paper.
In the above-described related art, the accuracy in identifying the recording medium can be increased by enlarging an imaging area of the recording medium. For example, the imaging area of the recording medium is enlarged by acquiring a surface image of the recording medium with a line sensor, which includes CMOS sensors serving as imaging devices arranged in line, while conveying the recording medium.
In this method, since the imaging area is large with respect to a light source, when the surface of the recording medium is imaged, an image area with an insufficient light quantity is formed by the influence of variations in the quantity of light emitted from the light source. If the type of the recording medium is identified by using this image area having an insufficient light quantity, the identification accuracy sometimes decreases.